1. Field of the Invention
The present invention relates generally to package sealing machinery of the band sealer type used for creating a seal across the top of a filled plastic bag in which heating and cooling bars are disposed on opposite sides of a pair of spaced, continuous, moving bands which are maintained to encompass drive and idler band wheels. The bands in turn transport bags to be sealed by the application of heat and pressure as they pass between the bars. More sequentially particularly, the present invention relates to controlling the tension of the bands by controlling the position of the band wheels about which the sealing bands move.
2. Description of the Related Art
Sealing machines of this class to which the present invention pertains employ a pair of continuous bands made of steel, fiberglass or other similar materials, which are arranged to travel in the same direction in confronting parallel relation with each other along a sealing run. The film laminae or other layers of plastic material of interest to be sealed together travel sequentially along the sealing run between the bands and the seal is made by a simultaneous application of controlled heat and pressure to the laminae as it is carried between the bands along the sealing path. If desired, additional support for the material to be sealed as by the provision of a pair of continuous chains also arranged to travel in the same direction in spaced confronting parallel relation and at the speed of the sealing bands. Generally, each bag, to be sealed is inserted at the machine's inlet end and, is pinched between the bands and transported past heating and cooling heat transfer members on opposite sides of the band through the machine to a discharge point which may connect to a continuous conveyor or other automated mechanism in a well-known manner.
At least one pair of bars or platens flanking the continuous bands are used to apply the heat and pressure to seal the laminae as it travels along the path. It will be appreciated that the heat transfer bars or platens must remain close to the travelling band but must make some allowance for aberrations in thickness of the laminae of film material to be sealed which travels therebetween. In addition, ample room has to be made available such that thicknesses of material even greater than anticipated, such as those caused by wrinkles or other multiple thicknesses does not produce jamming of the machine during high speed operation.
The jamming problem has been partially addressed by attempts to control the pressure exerted by the heat transfer bars and also by allowing a certain amount of relative pivotal motion of the bars to accommodate unusually thick specimens. However, prior to the present invention there has been no direct method of continuously controlling the tension of the belts in a manner which cooperates with the sealing process to aid in coping with aberrations in the material being processed. In addition, an easier way to change belts in the system has long been desired. In the past, the band wheels were mechanically cammed or held in place with springs which had to be released so that the wheel could be moved toward each other so that the old belt could be slipped off and the new slipped on. This was a time consuming operation which resulted in increased down time in normally high speed continuously operating machines.